1. Technical Field
The present invention relates to an electric power steering control system for controlling an electric power steering apparatus of an automobile and the like.
2. Background Art
In an electric power steering apparatus assisting a force required for a driver who manipulates a steering wheel of a vehicle, by means of an electric motor, it is very important that the electric motor is controlled so as to obtain an appropriate manipulation feeling of the steering wheel. Various electric power steering control systems for carrying out such control have been proposed up to now.
FIG. 11 is relevant to a block diagram of a conventional electric power steering control system shown in the Japanese Patent Publication (unexamined) No. 34469/1990. In addition, for the help of better understanding, FIG. 15 shows a construction of a generally known electric power steering apparatus. In FIG. 15, reference numeral 1 designates a steering wheel, and numeral 2 designates a torque sensor mounted on a shaft of the steering wheel. Numeral 10 designates a running wheel, and numeral 11 designates a gear mechanism that transmits a manipulation force of the steering wheel 1 to an angle control mechanism 12 of the running wheel 10.
In FIG. 11, numeral 2 designates a torque sensor mounted on the shaft of the steering wheel (numeral 1 in FIG. 15) for detecting the steering force of the driver. Numeral 3 designates a vehicle speed sensor for detecting a travelling speed of the vehicle (hereinafter referred to as a vehicle speed). Numeral 6 designates an electric motor (hereinafter referred to as a motor) for applying a steering assist force (hereinafter referred to as a steering assist amount or an assist force) to the shaft of the steering wheel (hereinafter referred to as a steering shaft). Numeral 7 designates a controller for computing the steering assist amount in response to a signal of the mentioned torque sensor 2 and vehicle speed sensor 3, and driving the motor 6.
Next, operation of the controller 7 is described. First motor current determining means 101 determines a first motor current based on an output of the torque sensor 2 and an output of the vehicle speed sensor 3, for example, based on a characteristic shown in FIG. 12. In FIG. 12, a motor current (approximate to a motor torque) is set so as to be generated when the steering torque exceeds a predetermined level, and further determined so as to be larger as the vehicle speed reduces.
A second motor determining means 102 approximately differentiates an output signal from the mentioned torque sensor 2 by torque differential operation means 104. The second motor determining means 102 multiplies the approximately differentiated value of the torque sensor output by a gain determined on the basis of the vehicle speed detected by the vehicle speed sensor 3. For example, as shown in FIG. 13, the greater the vehicle speed, the larger the gain is. Thus, the second motor current determining means 102 determines the second motor current in accordance with the characteristic as shown in FIG. 13. In case of FIG. 13, the motor current to be generated is in proportion to a rate of change of the steering torque (torque differentiation), and further determined so as to be larger as the vehicle speed becomes higher.
The above-described first motor current value and second motor current value are added together by an adder 30. By setting this addition value as a target current to be passed through the motor 6, the motor 6 is driven at a constant current by motor drive means 103.
In this manner, in the mentioned conventional electric power steering control system, the first motor current determining means 101 causes the torque to be generated for assisting the steering force of the driver. Further, the second motor current determining means 102 acts so as to cancel any effect due to inertial moment of the motor (i.e., a feeling of the driver that he feels the inertial moment of the motor as a weight when manipulating the steering wheel). As a result, the steering feeling will be improved.
Furthermore, when driving at a so high speed that the inertial moment of the motor 6 brings about any problem, the second motor current determining means 102 causes the gain, by which the above-described approximately differentiated value of the torque sensor is multiplied, to be large. On the other hand, when driving at a so low speed that the torque control system becomes unstable due to the second motor current and there is a possibility that the steering torque becomes vibrating, the second motor current determining means 102 causes the above-described gain to be small. In this respect, it is certain that, to prevent deterioration in feeling due to the motor inertia, the gain must be large enough. But, when the gain is too large, the gain by which the approximate differentiated value of the torque sensor is multiplied comes to be excessively large at a high speed travelling. Therefore, upon carrying out a quick steering such as lane change or avoidance of danger, the above-described second motor current comes to be exceedingly large thereby inducing the so-called deterioration in convergence such as an unstable feel of the steering wheel. Thus it is not always easy to adjust the gain.
Causes for producing the mentioned problems is hereinafter discussed further in detail. FIG. 13 is an example of the motor current determined by the second motor current determining means 102 as described above. FIG. 13 shows that the second motor current Idiff is calculated by multiplying together the mentioned approximate differentiation value of the torque and the above-described gain based on the vehicle speed. However, there is a time constant in this differential operation. Therefore, even after the driver loosens his grip on the steering wheel, (for extreme example, after the driver releases the steering wheel) thereby steering torque coming to be a value of nearly 0, the second motor current Idiff does not become 0 during a time in response to the time constant of the approximate differentiation of the torque. Accordingly, the current is still applied to the motor 6. In the case that this time constant is too long, the convergence of the vehicle gets worse, for example, at the time of lane change during driving at a high speed.
In the characteristic in FIG. 13, to prevent the steering feeling from getting worse due to the inertial moment of the motor 6 at a high speed travelling as described above, the above-described gain is set to be higher than that at a low speed. Such a setting means that the higher the speed, the larger the motor current flowing after the steering force is 0, and that the above-discussed convergence is induced to be getting worse.
To prevent such a condition, for example, on the contrary to the setting in FIG. 13, when setting the characteristic so that the gain reduces at a high speed, it becomes impossible to cancel the effect due to the motor inertial moment at a high speed travelling. As a result, the steering feeling gets worse. In this manner, there is a limit in controlling the steering feeling only by the gain, that is, by the adjustment of the inclination in FIG. 13.
To overcome such problems, the Japanese Patent Publication (unexamined) No. 157636/1998 discloses another construction of the electric power steering control system. FIG. 14 shows a construction similar to that disclosed in this patent publication.
In the known control system of FIG. 14, by means of a phase compensator 110, the approximately differentiated value of the torque sensor 2 is subject to a phase-compensation (so that frequency characteristic is improved, and a signal of an unnecessary frequency component of the approximately differentiated value is attenuated). The phase-compensated value is inputted to the second motor current determining means 102, and then in the same manner as in the foregoing conventional example in FIG. 11, the second motor current is computed. Addition of such a phase compensation circuit requires addition of any analog electronic circuit or digital circuit that requires extra software, eventually resulting in lowering of processing speed due to complicated processing.
In the conventional electric power steering control system of above-described arrangement, the gain becomes large by which gain the approximately differentiated value of the torque sensor is multiplied in travelling at a high speed. Accordingly, a problem exists in that the second motor current becomes excessively large at the time of carrying out a quick steering such as lane change or avoidance of danger thereby causing the so-called deterioration in convergence such as unstable feeling of the steering wheel.
Moreover, to overcome the mentioned problem concerning the convergence at a high speed travelling, on the contrary to the above-described conventional example, it was attempted to set the characteristic so that the gain, by which the approximately differentiated value of the torque sensor is multiplied at a high speed travelling, becomes small. In this attempt, however, another problem exists in that the inertial moment of the motor is not sufficiently compensated, eventually resulting in deterioration in steering feeling.
Furthermore, the other conventional system intending to solve the problem incidental to the conventional system by phase compensation requires a phase compensator. Thus, a problem exists in that addition of the electronic circuit is required in the case of the analog circuit forming such a phase compensator. Further, a problem exists in that in the case of employing a microprocessor serving as the phase compensator, since operation amount (i.e., volume to be computed) increases, an expensive microprocessor for carrying out the computing operation at a high speed is required resulting in a high cost.
The present invention has an object of providing an electric power steering control system in which, without increasing operation amount, convergence and steering feeling at a high speed driving are both improved while maintaining a light steering feeling at a low and medium speed driving, thereby securing a comfortable steering feeling.
For the purpose of obtaining an optimum steering feeling in response to the state of the vehicle, an electric power steering control system according to the invention is mounted on a vehicle that includes a steering apparatus, a torque sensor for detecting a steering force, a motor for assisting the steering force, and includes time constant variable type torque differential operation unit having a differential function for differentiating the output signal of the mentioned torque sensor and capable of setting a differential time constant of the mentioned differential function to a command time constant, and time constant determining unit for determining the differential time constant in response to a state quantity for the steering and commanding the differential time constant to the time constant variable type torque differential unit.
As a result, the electric power steering control system according to the invention has a comfortable steering feeling.
The foregoing and other object, feature, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.